1. Field of the Invention
The present invention relates to a cushioning rubber composition and a pneumatic tire.
2. Description of the Background Art
Global warming has recently continued due to the emission of carbon dioxide, and emissions of exhaust gas and carbon dioxide from motor vehicles are now being strictly regulated. Further, it is anticipated that light oil such as gasoline or diesel oil for motor vehicles obtained from petroleum resources is depleted in the future.
Therefore, lower fuel consumption is increasingly required to a motor vehicle such as a truck or a bus, and development of a tire having small rolling resistance (small fuel consumption) is regarded as important.
The rolling resistance of a tire can be effectively reduced by modifying a tread occupying the maximum mass among tire members, and has generally been reduced by modifying the tread. However, the tread is a tire member most influencing wear resistance and gripping performance of the tire, and hence reduction of the rolling resistance along with maintenance of the wear resistance and the gripping performance is technically reaching the limit.
Therefore, it is hereafter important to reduce the rolling resistance of the tire by modifying a tire member other than the tread.
A breaker cushion can be listed as the tire member to be modified for enabling reduction of the rolling resistance without influencing the wear resistance and the gripping performance of the tire. The breaker cushion, denoting a tire member 9 arranged between a carcass 6 and an end of a belt layer 7 in the axial direction of a tire shown in FIG. 1, is an important tire member remarkably influencing durability of the tire and the comfortableness of the motor vehicle.
In order to modify the breaker cushion, the loss tangent tan δ of a cushioning rubber composition is reduced by increasing the abundance ratio of a rubber component in the rubber composition thereby reducing the abundance ratio of a reinforcing filler such as carbon black, for example. In this case, however, the rubber composition is not sufficiently reinforced but the complex Young's modulus E* thereof is reduced, and hence the breaker cushion is easily deformed to increase energy loss. According to this method, therefore, the rolling resistance of the tire cannot be reduced.
According to another method, the loss tangent tan δ of the rubber composition is reduced by adding a large quantity of vulcanizer to the cushioning rubber composition. In this case, the rolling resistance can be reduced without reducing the abundance ratio of the reinforcing filler in the rubber composition. However, the physical properties of the rubber composition are remarkably changed by heat aging resulting from hard use of the tire, and the tire has insufficient resistance (breakage resistance) for withstanding an impact against the side surface of the tire upon running up onto a curbstone or traveling on an irregular ground when the rubber composition is employed as the material for the breaker cushion.
The level of the breakage resistance required to the tire varies with the type of the tire, and a radial tire employed for a vehicle such as a heavy-duty truck or a bus receiving a heavier load than a small truck requires superior breakage resistance.
Japanese Patent Laying-Open No. 2004-099804 describes a rubber composition for a tire containing carbon black having a nitrogen adsorption specific surface area of 30 to 70 m2/g, a dibutyl phthalate oil absorption of at least 134 cm3/100 g and an iodine adsorption number of 20 to 50 mg/g as a rubber composition for a tire excellent in balance between reinforcing property and low rolling resistance (LRR).
Japanese Patent Laying-Open No. 2000-71716 describes a technique of arranging a reinforcing rubber layer between a belt ply and a carcass and setting the reinforcing rubber layer to prescribed physical properties in order to reduce fuel consumption of a vehicle by reducing the mass of a tire while keeping strength of the tire.
However, further improvement of performance is required to each technique.